This invention relates to suppressed carrier modulation systems and more particularly a modulation system capable of providing a modulator having a small carrier leak and small higher harmonic component and a wide dynamic range.
One example of a prior art suppressed carrier modulation system will first be described with reference to FIG. 1 of the accompanying drawing which comprises an input terminal IN.sub.1 supplied with a modulation signal, another input terminal IN.sub.2 supplied with a signal to be modulated and an output terminal OUT for producing a modulated output signal. There are provided a transistor Q.sub.1 having a base electrode connected to the input terminal IN.sub.1, a transistor Q.sub.2 with its base electrode connected to a source of DC voltage E.sub.1 and its emitter electrode connected to the emitter electrode of transistor Q.sub.1 via a resistor R.sub.1, and transistors Q.sub.3 and Q.sub.4 which constitute a constant current source for the transistors Q.sub.1 and Q.sub.2 with their base electrodes connected to a source of DC voltage E.sub.2 and emitter electrodes grounded respectively through resistors R.sub.3 and R.sub.4. There are also provided transistors Q.sub.5, Q.sub.6 and Q.sub.7, Q.sub.8 with their emitter electrodes connected with each other. The base electrodes of transistors Q.sub.5 and Q.sub.8 are connected in common to the input terminal IN.sub.2 while commonly connected emitter electrodes of transistors Q.sub.5, Q.sub.6 and Q.sub.7, Q.sub.8 are connected to the collector electrodes of transistors Q.sub.1 and Q.sub.2, respectively. The base electrodes of transistors Q.sub.6 and Q.sub.7 are connected in common to one pole of a source of DC voltage E.sub.3 whereas the collector electrode of transistor Q.sub.6 is connected to the collector electrode of transistor Q.sub.8 and to the output terminal OUT. A source of supply V.sub.cc is connected directly to the collector electrodes of transistors Q.sub.5 and Q.sub.7 and is also connected to the collector electrode of transistor Q.sub.8 via a resistor R.sub.2.
When a modulation signal is supplied to the base electrode of transistor Q.sub.1, and when a signal to be modulated is supplied to the base electrodes of transistors Q.sub.5 and Q.sub.8, a modulated output signal is derived out from the collector electrodes of transistors Q.sub.6 and Q.sub.8.
The modulation system having a construction described above, however, encounters a problem that the carrier leak, i.e., the leakage of the carrier occurring when the modulation signal supplied to the input terminal IN.sub.1 is zero becomes large. The carrier leak is determined by the difference in the collector currents of transistors Q.sub.1 and Q.sub.2 and a load resistance so that there are many causes that increase the carrier leak, such as the difference in the base-emitter voltages V.sub.BE of transistors Q.sub.3 and Q.sub.4 which constitute the constant current source, the difference in the resistance ratio between emitter resistors R.sub.3 and R.sub.4 of transistors Q.sub.3 and Q.sub.4, the difference in the base bias voltage of transistors Q.sub.1 and Q.sub.2 and the difference in the base-emitter voltages of transistors Q.sub.1 and Q.sub.2. Therefore, such a modulation system requires sophisticated adjustment and can not be used satisfactorily. Moreover, since the transistors are connected in cascade of three stages, the dynamic range of the modulation system is narrow and it is difficult to operate it with low source voltage.